Field coil



FIG. 1 is a perspective view of a field coil showing my new design;

FIG. 2 is a side elevational view thereof; the other side elevational views being substantially identical thereto;

FIG. 3 is a top plan view thereof, the bottom plan view being identical thereto; and,

FIG. 4 is a perspective view showing the field coil in use; the broken lines depict environmental structure for illustrative purposes only and form no part of the claimed design. 

I claim the ornamental design for a field coil, as shown and described. 